Liposomes are fatty or oily globules occurring in cytoplasm. The term "synthetic liposomes" refers to microscopic globules, having a maximum diameter of the order of 10,000 A and preferably having a diameter between 300 and 2,000 A, bounded by a wall formed by at least one bimolecular layer (having a thickness of the order 3,200 A) of a compound of the general formula XY, where X is a hydrophilic polar group and Y is a hydrophobic non-polar group, the globules containing an aqueous liquid, for example an aqueous solution of at least one biologically active substance, and existing generally in the form of a colloidal dispersion in an aqueous medium such as an aqueous saline solution, in particular a 0.9% by weight sodium chloride solution.
The preparation of liposomes provides a method of encapsulation which is most practical and effective for aqueous liquids and which is particularly useful for administration of biologically active substances, particularly medicaments, into living organisms, while avoiding the destruction or inactivation of the substance in the organism, for example by the action of gastric or intestinal juices, before the substances reach the site where they are required to act.
By selection of the compound of formula XY used to form the wall of the liposomes, it is possible to produce liposomes having walls which resist the activity of certain zones in the organism and are only attacked in the presence of particular agents which only exist in the organs where the biologically active substance is to be liberated.
Two processes for the preparation of liposomes are known.
One of these processes consists of placing a lipid in contact with an aqueous liquid which it is wished encapsulate and then warming the heterogeneous mixture thus obtained at a temperature slightly above ambient temperature and then submitting the mixture to vigorous agitation following ultrasonic vibration.
The other process consists of dissolving a compound of formula XY (where X and Y are as defined above), for example a lipid, in a volatile solvent, forming a film of the compound on the walls of a receptacle by evaporating the solvent from the solution thus obtained, introducing in the same receptacle the liquid which is wished to encapsulate in the liposomes, and finally submitting the liquid in the receptacle to the action of ultrasonic vibrations.
The two processes thus require the use of a total volume of the liquid which it is desired to encapsulate very much larger than the volume of that liquid which is finally contained in the liposomes produced by the process. According to these processes, the liposomes are formed in effect in the state of a colloidal dispersion of globules in a liquid phase which comprises the fraction of the liquid to be encapsulated which has not been retained in the interior of the liposomes. The ratio of the volume of encapsulated liquid in the interior of the liposomes to the total volume of the surrounding liquid is in general of the order of 1 to 10%.
In consequence, if the liquid to be encapsulated has a high value, as is the case most generally met when the liquid is a solution of a biologically active substance, it is necessary to recover the fraction of that liquid which has not been encapsulated before using it in further operations to form liposomes. This recovery requires the separation of the liposomes from the liquid, then purification of the liquid itself and, usually, the readjustment of the concentration of the active substance. In practice the separation and purification steps require the use of large volumes of solvents and in consequence the concentration of the liquid containing the active substance has to be adjusted.
The necessity of carrying out the steps of purification and the readjustment of the concentration of the liquid containing the active substance render these two processes difficult to put into practice on an industrial scale.